Vehicle control systems are known to the art which are characterized by a centralized controlling authority which receives information from the vehicles within its zone of control, and in which the controlling authority transmits signals for the purpose of safely controlling the travel of the vehicles. Essential to any such system is, of course, an effective communication system, for it is only through the communication system that the controlling authority can determine vehicle location and speed, and it is only through such facility that the controlling authority's commands can be communicated to the vehicles for purposes of controlling their travel. Examples of control systems of this sort in which the vehicles are railroad vehicles and the controlling authority includes a computer or computer complex are found in U.S. Pat. Nos. 3,268,727; 3,819,932; and 3,794,834.
Typically required in a practical system is the capability for the controlling authority, and its attendant communication channels, to handle approximately 40 vehicles simultaneously. The communication to and from these vehicles can be handled on a time multiplexed basis in which the controlling authority periodically transmits a message to each of the vehicles within its control zone, and reception of the message aboard the vehicle generates a reply message from the vehicle. As will be apparent to those skilled in the art, the length of the communication cycle may have a substantial impact on the minimum vehicle headway. For example, in a practical situation, the communication cycle may be on the order of 1/2 second. Because of this, if the controlling authority determines that motion by a particular vehicle should be terminated, it can be up to 1/2 second before that command may be communicated to the vehicle. As a backup for the communication system, the vehicles normally carry a timer which is reset upon each receipt of a message from the controlling authority so that if a communication channel fails, the vehicle will be automatically stopped. Because of difficulties in designing fail-safe timers with precise timing periods, it may be up to a full second after the controlling authority determines a vehicle is to be stopped, before the vehicle enters the stopping mode. These considerations obviously bear upon minimum system headway.
Attempts to decrease the communication cycle time require corresponding increase in communication bandwidth or decrease in the amount of information to be communicated.
It is therefore one object of the present invention to decrease control system delays caused by the communication system. It is another object of the present invention to minimize the bandwidth required for communications by such a control system.
It is another object of the invention to facilitate communications in such system by removing unnecessary communication.